ABSTRACT Noninvasive assessment of fetal cardiac electrophysiology has not been routinely possible despite many decades of fetal ECG research. Recently, however, we and other groups have demonstrated the utility of fetal magnetocardiography (fMCG), the magnetic analog of fetal ECG, as an effective new method of assessing fetal heart rate, rhythm, and conduction. The long-term goal of this program is to develop fMCG as a new technology for fetal investigation and clinical application. If successful, the research can improve the diagnosis of life-threatening forms of fetal arrhythmia and shed light on causes of unexplained fetal death. The last few years have seen remarkable progress in demonstrating the diagnostic efficacy of fMCG. Crucial progress has also been made toward the development of fMCG technology. During the next project period, we will focus on several areas that are critical to overcoming the obstacles to clinical adoption of fMCG. FMCG is still a very new area. Additional human studies are needed to better define and further expand the role of fMCG for clinical management of fetal long QT syndrome and other life-threatening fetal arrhythmia. Optically-pumped magnetometers (OPMs) represent an important breakthrough that can overcome a major barrier to clinical adoption of fMCG by making the technique more practical and cost-effective. The specific aims are as follows: 1. Perform serial studies of fetuses at risk of LQTS. Little is known about the development of LQTS prior to birth. The studies are needed to ascertain how early symptoms manifest, follow the progression of the disease, and help determine the frequency and timing of fMCG sessions for optimal clinical management of fetal LQTS. 2. Continue our studies of pregnancies complicated by fetal arrhythmia and other high-risk pregnancy conditions. The proposed studies will help define the contribution of fMCG for clinical management of a range of serious fetal arrhythmia, including some that have not been previously studied. 3. Demonstrate the efficacy of OPMs as a new technology for fMCG. We will directly compare recordings made with an OPM system and an FDA-cleared SQUID magnetometer.